The term graphene designates a one atom thick planar sheet of sp2-hybridized rings with 6 carbon atoms. Perfect graphenes consist exclusively of hexagonal cells. Cylindrical graphene layers are termed carbon nanotubes. The term graphene may also be used when features of single layers in graphite are discussed. Such features are, e.g., reactivity or undergone reactions, respectively, or structural relations.
Graphene layers may be produced by suitable abrasion, mechanical exfoliation or chemical vapour deposition. One such method is the so-called Scotch-Tape-Method [10]. In this method single layers are removed from a graphite crystal and transferred to a sample holder.
In a recently described chemical vapour deposition method is disclosed, wherein, prior to being able to generate a graphene layer, a SiO2/Si substrate has to be covered with a thin Ni layer, and this coated substrate has to be subjected to a specific gas treatment. For making the graphene layer usable to coat other substrates, the Ni or the SiO2 layer has to be dissolved. Such graphene films had very good electrical, optical and mechanical (e.g. bending) properties [9].
Another method is to heat silicon carbide to high temperatures (1100° C.) to reduce it to graphene. This process produces a layer, the extent of which is dependent on the size of the SiC substrate used, and, due to the expensive starting material, is quite expensive and also limited in use due to the high temperature needed.
Graphene is quite different from most solids. Graphene behaves as a semi-metal or “zero-gap” semiconductor and has a remarkably high electron mobility at room temperature.
Aqueous dispersions of carbonaceous material, such as graphite, graphene or carbon nanotubes, are described in the literature. The production of aqueous graphite dispersions from graphite with preferred particle sizes between 1 μm and 50 μm or 100 μm, respectively, stabilized by various dispersants, has been described (see e.g., U.S. Pat. No. 5,476,580 and WO2007/031055). Up to 20% by weight or up to 70% by weight, respectively, graphite may be dispersed in water.
Dispersions of carbon nanotubes with a nanotubes content of 2% are, e.g., obtainable by stabilization with the dispersing aid polyethylene glycol [1] or via chemical functionalizing of the carbon nanotubes [2].
WO2008/048295 describes a method for stabilizing graphene layers in a solvent by means of polymer coating. An about 0.065% by weight graphene based material is obtained. The colloidal graphene dispersion is provided by reduction of dispersed graphite oxide using hydrazine hydrate.
Dan Li et al. [3] describe that the aqueous solution may be electrostatically stabilized by ammonia resulting in a graphene based material with graphene content of about 0.015% by weight. Also Dan Li et al. prepared the colloidal graphene dispersion from a graphite oxide dispersion by reduction with hydrazine hydrate. The reduction with hydrazine hydrate as disclosed in the state of the art results in a C/O ratio of below 13.5, meaning that at most about 80% of the oxygen has been removed [4, 5, 6].
Another method for reduction of graphite oxide is thermal reduction. Dependent on the desired production conditions, purity conditions and reduction conditions, the thermal reduction of graphite oxide powder is slow up to a temperature of about 200° C., and then becomes boisterous [7]. Reduction at this temperature results in an elimination of about 65% of the oxygen, 10% of the carbon, and most of the hydrogen, due to the formation of CO, CO2 and water. Heating to higher temperatures results in continuous further reduction. A temperature of about 1000° C. is required for removal of about 90% of the oxygen. Thus produced graphite material can no longer be dispersed in water to form a colloidal dispersion.
Hence, it is a general object of the present invention to provide a method for producing stable colloidal dispersions of graphene in solution, in particular, colloidal graphene dispersions that do not need any dispersant.
It is also an object of the present invention to provide stable dispersions of single and multiple graphene layers.
It is also an object of this invention to provide uses of such graphene dispersions.
It is a further object of the invention to provide a multi-graphene with improved intercalating features that may, e.g., advantageously be used for the production of electrodes in rechargeable lithium ion batteries.